xen/netfront: fix crash when removing device

When removing a netfront device directly after a suspend/resume cycle
it might happen that the queues have not been setup again, causing a
crash during the attempt to stop the queues another time.

Fix that by checking the queues are existing before trying to stop
them.

This is XSA-465 / CVE-2024-53240.

Reported-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Fixes: d50b791 ("xen-netfront: Fix NULL sring after live migration")
Signed-off-by: Juergen Gross <jgross@suse.com>

drivers/net/xen-netfront.c

@@ -867,7 +867,7 @@ static netdev_tx_t xennet_start_xmit(struct sk_buff *skb, struct net_device *dev
 static int xennet_close(struct net_device *dev)
 {
 	struct netfront_info *np = netdev_priv(dev);
-	unsigned int num_queues = dev->real_num_tx_queues;
+	unsigned int num_queues = np->queues ? dev->real_num_tx_queues : 0;
 	unsigned int i;
 	struct netfront_queue *queue;
 	netif_tx_stop_all_queues(np->netdev);
@@ -882,6 +882,9 @@ static void xennet_destroy_queues(struct netfront_info *info)
 {
 	unsigned int i;
 
+	if (!info->queues)
+		return;
+
 	for (i = 0; i < info->netdev->real_num_tx_queues; i++) {
 		struct netfront_queue *queue = &info->queues[i];